Because rotary regenerators for gas turbine engines are now being made of a ceramic material, they require seals formed from material that can provide sufficient coating wear life. To achieve this wear life, this material must be oxidation resistant at temperatures up to and exceeding about 1600.degree. F., and have a low coefficient of friction to minimize torque loads on the regenerator.
Typically, these seals have multi-layer coatings with a surface layer of a high temperature, ceramic solid lubricant. For example, U.S. Pat. No. 3,481,715 discloses a regenerator seal having a surface layer comprised of nickel oxide, calcium fluoride, and calcium oxide. Another common surface layer is comprised of zinc oxide, calcium fluoride, and tin oxide. A common problem with these surface layers is their inability to accommodate large mismatch strains due to thermal expansion, distortions of the metallic substrate resulting from pressure induced deflection, and seal creep (permanent deformation). Additionally, experimental test results suggest that these solid lubricant ceramic coatings may sinter (densify) during high temperature operation, thus contributing further to strain mismatch problems. All of the above strain mismatch scenarios can lead to premature failure of the ceramic surface layer. Of particular concern is the possibility of massive spalling which can lead to damage and subsequent failure of both the seal and the ceramic regenerator core.
U.S. Pat. No. 4,914,794, which is assigned to the assignee of this application and shares a common inventor, discloses a method for making an abradable ceramic, (i.e. stabilized zirconia), turbine shroud coating strain tolerant by plasma spraying or other line of sight deposition process to form shadow gaps that result in a segmented morphology. Attempts to apply this method of segmentation to a regenerator seal's ceramic, solid lubricant coating was not successful primarily due to the lower melting points of the solid lubricant coating relative to the abradable turbine shroud stabilized zirconia coatings. The lower melting point of the ceramic solid lubricant coatings resulted in increased molten particle flow during plasma spraying, which resulted in the shadow gaps being bridged. The resulting coating exhibited poor thermal strain tolerance, with coatings failing prematurely under cyclic thermal exposure.
Accordingly there is a need for a method of forming a segmented morphology in a solid lubricant ceramic coating having lower melting temperatures than stabilized zirconia abradable shroud coatings.